1. Field of the Invention
This invention relates generally to the processing of wafers within semiconductor processing chambers and, more particularly, to a wafer holder that effects a more uniform temperature distribution across a wafer held thereon.
2. Description of the Related Art
High-temperature ovens, or reactors, are used to process semiconductor wafers from which integrated circuits are made for the electronics industry. A substrate, typically a circular silicon wafer, is placed on a wafer holder. If holder helps to attract heat, it is called a susceptor. The wafer and wafer holder are enclosed in a quartz chamber and heated to high temperatures, such as 600–1200° C. or even higher, by a plurality of radiant lamps placed around the quartz chamber. In one process, a reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits. In recent years, single-wafer processing has grown for a variety of reasons, including its greater precision as opposed to processing batches of wafers at the same time, while larger diameter wafers are employed to compensate for reduced throughput as compared to batch processing.
Various CVD process parameters must be carefully controlled to ensure the high quality of the resulting semiconductor. One such parameter is the temperature profile across the wafer during processing. The deposition gas reacts at particular temperatures and deposits on the wafer. If the temperature varies greatly across the surface of the wafer, uneven deposition of the reactant gas occurs. Similarly, temperature uniformity can be important for a variety of other semiconductor fabrication processes, such as etching, annealing, doping, etc.
A variety of different types of wafer holders exist. In one design, an upper surface of the wafer holder includes a wafer-receiving pocket or recess sized and adapted to receive a wafer of a particular size. The recess is defined by an outer shoulder circumscribing an inner pocket surface that is lower than the shoulder's upper surface. The upper surface of the wafer holder may include a grid formed by a plurality of intersecting grooves in the inner pocket surface. By permitting gas flow around the edges of and underneath the wafer, the grooves facilitate wafer pickup from the surface of the wafer holder. The grooves thereby avoid suction between the wafer and the wafer holder and thereby prevent the wafer from sticking to the wafer holder. Similarly, the channels inhibit wafer slide during wafer drop off, permitting gas to compress underneath and escape around the wafer edges without forming a cushion. The wafer holder is typically formed of graphite and coated with silicon carbide, which is relatively inert, durable, absorbs radiant heat and helps to prevent graphite particles from flaking off of the wafer holder and contaminating the reactor environment. Advantageously, silicon carbide has an ultra high purity.
A typical process of manufacturing a wafer holder involves forming the part from graphite, possibly with the wafer-receiving recess and/or grid structure therein. The part is then heated to a temperature as high as 1600–1700° C., to remove impurities (metals, etc.) and/or gases from within the graphite, and then permitted to cool. Then, in a separate high temperature process, the part is coated with silicon carbide and subsequently annealed. Advantageously, the coating closely conforms to and seals the graphite after outgassing.
During the cooling step, it has been found that wafer holders having certain configurations tend to form slight concavities on one surface. This is often referred to as “dishing.” In particular, if a wafer holder has a larger surface area on its upper surface (on which the wafer is held) than its lower surface, a concavity will form within the upper surface. One type of wafer holder that exhibits this property is the aforementioned design having a grid structure in its upper surface and a generally flat bottom surface. In this design, a concavity is formed in the upper surface of the wafer-receiving pocket. The concavity is caused by a differential in thermal expansion between the graphite core and the silicon carbide coating. In particular, the coefficient of thermal expansion of silicon carbide is greater than that of graphite. If the silicon carbide had a lower coefficient of thermal expansion, the wafer holder would dish or bow in the opposite direction. One advantage of using a ceramic coating material, such as silicon carbide, having a coefficient of thermal expansion larger than that of graphite is that, in the finished wafer holder, the ceramic coating is in a state of compression. Ceramic materials are known to be stronger in compression.
One problem that has been encountered is that wafer holders tend to assume a “saddle” shape during manufacturing and/or wafer processing. Saddling is a form of dishing in which the formed concavity is not uniform. In a saddled wafer holder, the upper surface of the outer shoulder is not flat. Rather, it has opposing high points and low points, such as the edge of a potato chip. In other words, a “saddle shape” is one having two high edges and two low edges on a surface. Saddling of the wafer holder tends to result in temperature non-uniformities across the wafer surface.
In order to avoid the concavity, some manufacturers have attempted to minimize the surface area differential by forming an additional grid structure on the bottom surface of the wafer holder. See, e.g., U.S. Pat. No. 5,403,401, issued Apr. 4, 1994 to Haafkens et al. Such a bottom side grid can interfere thermally and mechanically with normal operation of the susceptor in the reactor, and moreover doubles the amount of fine machining required for production.
Accordingly, a need exists for improved susceptors and processes of forming the same that avoid the advent of saddling.